1. Field of the Invention
The present invention relates to a vane pump used as a hydraulic source of a power steering apparatus, more particularly, to such a vane pump in which ability in projection of vanes is enhanced when a rotation of a rotor of the pump is started.
2. Description of the Related Art
Conventionally, there is of such a vane pump described in Japanese Patent Publication (Kokai) No. 9-32740 (1997).
In such a conventional vane pump, it is composed of a rotor rotatably disposed in a cam ring, plural vanes projectably provided in corresponding slits formed in the rotor, and a pair of sidewall members (an end surface of a body and a side plate) provided to close both side portion of the cam ring. With this configuration, a pump operation is performed with rotation of the rotor as the vane slidably contacts its top portion and its side surfaces with an inner surface of the cam ring and with surfaces of the side wall members facing with the vane.
In such a vane pump, it is appeared such a phenomenon that the pump operation is not started immediately when the rotation of the rotor of the pump is started, because the vane is not sufficiently projected from the slit of the rotor.
After the pump has been driven, the vane is projected until its top portion contacts the inner surface of the cam ring by discharged pressure of the pump that is introduced into a backpressure groove formed in each of the sidewall members. However, just after the rotation of the rotor of the pump is started in a condition that the pumping operation is not started, only a centrifugal force upon rotation of the rotor contributes on the projection of the vane, so that the projection thereof cannot sufficiently performed in such a condition.
As a result of an analysis of this phenomenon in the applicant, when a vane 60 in slit 510 formed in a rotor 50 is rotated with the rotor 50, pressing force "F" that presses the vane 60 onto one of inside walls of the slit 510 (backward in a rotational direction of the rotor 50) acts on the vane 60, as shown in FIG. 1. Therefore, the projection of the vane 60 is prevented by this pressing force "F". As a result of further analysis, it is discovered such that the phenomenon that the vane 60 is pressed onto the inside wall of the slit 510 is caused by slide resistances of the side surface of the vane 60 with inside surface of the both sidewall members.